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C#:.NET:Collections

ArrayList. This collection dynamically resizes. It grows in capacity as elements are added (if space is needed). It is most often used in older C# programs.


It holds objects. It stores elements of type object—casting is needed. We end up with as-casts, numeric casts to test elements. Code becomes messy.

Object

Add. This method appends a new element object to the end of the ArrayList. We can keep adding elements to the collection until memory runs out. The objects are stored in the managed heap.

Here: We see a complete console program. When you run it, three elements are added to the ArrayList.

Elements: The first element is a string containing "One." The last element is "Three."

Based on:

.NET 4.5

C# program that uses ArrayList

using System.Collections;

class Program
{
    static void Main()
    {
	//
	// Create an ArrayList and add three elements.
	//
	ArrayList list = new ArrayList();
	list.Add("One");
	list.Add("Two");
	list.Add("Three");
    }
}

Arguments. It is easy to pass ArrayList as an argument. However, in the receiving function, you have to know (or find out) the type of each element.

Next: In this program we pass the ArrayList as an argument to the Example() method.

C# program that uses ArrayList parameter

using System;
using System.Collections;

class Program
{
    static void Main()
    {
	//
	// Create an ArrayList and add two ints.
	//
	ArrayList list = new ArrayList();
	list.Add(5);
	list.Add(7);
	//
	// Use ArrayList with method.
	//
	Example(list);
    }

    static void Example(ArrayList list)
    {
	foreach (int i in list)
	{
	    Console.WriteLine(i);
	}
    }
}

Output

5
7

Return ArrayList. You can also use the ArrayList as a return value. It is usually best to reuse the same ArrayList instead of combining more than one.


Combine. We can use AddRange to combine two ArrayLists. Internally, AddRange uses the Array.Copy or CopyTo methods, which have better performance than some loops.

Array.Copy

Here: The first ArrayList has two elements added to it. Next, the second ArrayList has two elements added.

Then: The second ArrayList is appended to the first using the AddRange method. The example finally shows the output.

Console.WriteLine
C# program that uses Add and AddRange

using System;
using System.Collections;

class Program
{
    static void Main()
    {
	//
	// Create an ArrayList with two values.
	//
	ArrayList list = new ArrayList();
	list.Add(5);
	list.Add(7);
	//
	// Second ArrayList.
	//
	ArrayList list2 = new ArrayList();
	list2.Add(10);
	list2.Add(13);
	//
	// Add second ArrayList to first.
	//
	list.AddRange(list2);
	//
	// Display the values.
	//
	foreach (int i in list)
	{
	    Console.WriteLine(i);
	}
    }
}

Output

5
7
10
13

Count. This is a virtual property. When you use Count, no counting is actually done. Instead a cached field value is returned. This means that Count is fairly fast.

Here: The example shows the Count property. It also shows the Clear method, and how this affects the count.

Int: The Count property returns an int. This will always be a positive value. No calculation takes place in the property itself.

Int

Clear: You can call the instance method Clear on your ArrayList. Internally, this calls the Array.Clear method.

Array.Clear

Sometimes: Code is clearer if you instead create a new ArrayList. This may also affect performance.

C# program that uses Count

using System;
using System.Collections;

class Program
{
    static void Main()
    {
	//
	// Create an ArrayList with two values.
	//
	ArrayList list = new ArrayList();
	list.Add(9);
	list.Add(10);
	//
	// Show number of elements in ArrayList.
	//
	Console.WriteLine(list.Count);
	//
	// Clear the ArrayList.
	//
	list.Clear();
	//
	// Show count again.
	//
	Console.WriteLine(list.Count);
    }
}

Output

2
0

Sort, reverse. Many dynamic arrays (such as ArrayList) must be frequently sorted. We call the instance Sort method and then Reverse. These methods work in-place.

Tip: You can sort subsets (ranges) of elements in your ArrayList using the third overload. This is useful in rare situations.

Also: You can Reverse only a range of your ArrayList. This is useful even less often.

Note: The Sort method in the base class libraries is an instance method (with no parameters) on ArrayList.

Elements: The ArrayList Sort method works on different element types. The example here shows strings.

C# program that sorts ArrayList and reverses

using System;
using System.Collections;

class Program
{
    static void Main()
    {
	//
	// Create an ArrayList with four strings.
	//
	ArrayList list = new ArrayList();
	list.Add("Cat");
	list.Add("Zebra");
	list.Add("Dog");
	list.Add("Cow");
	//
	// Sort the ArrayList.
	//
	list.Sort();
	//
	// Display the ArrayList elements.
	//
	foreach (string value in list)
	{
	    Console.WriteLine(value);
	}
	//
	// Reverse the ArrayList.
	//
	list.Reverse();
	//
	// Display the ArrayList elements again.
	//
	foreach (string value in list)
	{
	    Console.WriteLine(value);
	}
    }
}

Output

Cat
Cow
Dog
Zebra

Zebra
Dog
Cow
Cat

Sort, internals. Internally, the Sort method ends up in an internal TrySZSort or QuickSort method when it doesn't throw an exception.

Note: TrySZSort is optimized for one-dimensional arrays, also known as "Zero" arrays or vectors.


Sort, performance. The TrySZSort method used in the base class libraries is implemented in native code. It has been heavily optimized.

Thus: Using Sort on ArrayList or on Array is faster than most custom implementations.


Insert, Remove. Here we insert and remove elements in an ArrayList. We see the RemoveAt method for erasing a single element, and then Insert and RemoveRange.

Tip: The first argument to Insert is the position: this is equivalent to the index of the element.

C# program that uses Insert and Remove

using System;
using System.Collections;

class Program
{
    static void Main()
    {
	//
	// Create an ArrayList with three strings.
	//
	ArrayList list = new ArrayList();
	list.Add("Dot");
	list.Add("Net");
	list.Add("Perls");
	//
	// Remove middle element in ArrayList.
	//
	list.RemoveAt(1); // It becomes [Dot, Perls]
	//
	// Insert word at the beginning of ArrayList.
	//
	list.Insert(0, "Carrot"); // It becomes [Carrot, Dot, Perls]
	//
	// Remove first two words from ArrayList.
	//
	list.RemoveRange(0, 2);
	//
	// Display the result ArrayList.
	//
	foreach (string value in list)
	{
	    Console.WriteLine(value); // <-- "Perls"
	}
    }
}

Output

Perls

Loop. The for-loop is popular and useful. We need to cast elements after accessing them. The [i] part in the example below demonstrates how to use the indexer on the ArrayList.

Indexer

Cast: The "as" cast in C# is probably the best way to cast reference types such as string.

As

Tip: After you cast, you can check the result for null before using the variable, to see if the cast succeeded.

C# that uses ArrayList and for

using System;
using System.Collections;

class Program
{
    static void Main()
    {
	//
	// Create an ArrayList with three strings.
	//
	ArrayList list = new ArrayList();
	list.Add("man");
	list.Add("woman");
	list.Add("plant");
	//
	// Loop over ArrayList.
	//
	for (int i = 0; i < list.Count; i++)
	{
	    string value = list[i] as string;
	    Console.WriteLine(value);
	}
    }
}

Output

man
woman
plant

GetRange. This will return a subset of the original ArrayList in a new ArrayList. This is ideal when you know a certain part of your ArrayList has a different purpose or behavior.

SetRange: The SetRange method on ArrayList is also useful when you need to replace a range.

However: I have not found SetRange to be useful, as often you will just want to replace elements in a for-loop.

C# that uses GetRange

using System;
using System.Collections;

class Program
{
    static void Main()
    {
	//
	// Create an ArrayList with 4 strings.
	//
	ArrayList list = new ArrayList();
	list.Add("fish");
	list.Add("amphibian");
	list.Add("bird");
	list.Add("plant");
	//
	// Get last two elements in ArrayList.
	//
	ArrayList range = list.GetRange(2, 2);
	//
	// Display the elements.
	//
	foreach (string value in range)
	{
	    Console.WriteLine(value); // bird, plant
	}
    }
}

Output

bird
plant

IndexOf. The IndexOf and LastIndexOf methods on ArrayList are similar to those on strings. You pass in the value you are looking for, the start index, the number of elements to search.

Info: IndexOf will return -1 if the element could not be located. This value must be specially tested.

IndexOf

Convert. Arrays offer more performance and compatibility, so you will want to convert ArrayLists to arrays. We use casting to convert an ArrayList to an array.

Convert ArrayList, Array

Benchmark, List, ArrayList. Here we compare the performance of List (a generic collection) to ArrayList. We add, remove, test and clear each collection.

Remove: This is done by value, so an internal search occurs. The if-statements just test the last element.

Result: List performs with more than twice the speed of ArrayList. It performs this simple benchmark faster.

Unboxing: There is a performance penalty in using ArrayList, particularly on value types. This is because boxing (and unboxing) occurs.

Unboxing
C# that benchmarks List, ArrayList

using System;
using System.Collections;
using System.Collections.Generic;
using System.Diagnostics;

class Program
{
    static void Main()
    {
	List<int> list = new List<int>();
	ArrayList array = new ArrayList();

	const int max = 1000000;
	var s1 = Stopwatch.StartNew();
	// Version 1: use List.
	for (int i = 0; i < max; i++)
	{
	    list.Add(10);
	    list.Add(20);
	    list.Add(30);
	    list.Remove(30);
	    if (list[list.Count - 1] != 20)
	    {
		return;
	    }
	    list.Clear();
	}
	s1.Stop();
	var s2 = Stopwatch.StartNew();
	// Version 2: use ArrayList.
	for (int i = 0; i < max; i++)
	{
	    array.Add(10);
	    array.Add(20);
	    array.Add(30);
	    array.Remove(30);
	    if ((int)array[array.Count - 1] != 20)
	    {
		return;
	    }
	    array.Clear();
	}
	s2.Stop();
	Console.WriteLine(s1.Elapsed.TotalMilliseconds);
	Console.WriteLine(s2.Elapsed.TotalMilliseconds);
	Console.Read();
    }
}

Results

 67.4821 ms,    List      add, remove, test, clear
151.8429 ms,    ArrayList add, remove, test, clear

List versus ArrayList. It is usually better to use List. But older applications may be using ArrayList. And it is often best not to have to rewrite them.

List

Note: Lists not only avoid boxing or unboxing, but they also lead to clearer and less bug-prone code.

And: With List, the compiler can check your code for type integrity before runtime. This improves reliability.


BinarySearch. This method implements the binary searching algorithm. This uses a "divide and conquer" approach to finding the correct element. It only works on presorted data.

Thus: Never use BinarySearch if your ArrayList might not be already sorted. The results could be invalid.

BinarySearch List

A summary. ArrayList is a collection that is best avoided. But it is often used in older legacy programs—so it must be supported. Newer .NET Framework versions offer better collections.